The Master Brain appeared as a futuristic globe filled with webbing that slowly rotated above the Master. It was able to alter a person's imagination, removing it altogether and was capable of altering perceptions of reality within the Land.
Originally, the Master Brain had enslaved a writer from 1926 as the Master and made him write until he grew old. The Master tried to replace himself with the Second Doctor so that he could leave, with the Master Brain controlling his mind and speaking through him in third person. It planned to take over Earth by turning Mankind into a "string of sausages," all the same, without free will. The Doctor freed the Master and the Master Brain was destroyed by Jamie and Zoe overloading it. (TV: The Mind Robber)
After pulling a Cyberman invasion force into the Land, Zoe Heriot rebuilt the Master Brain and became the Mistress of the Land in order to fight the Cybermen. They sought to use the computer to remove imagination from Humanity so they would be docile and conquerable, but were turned into fiction and winked out of existence. (AUDIO: Legend of the Cybermen) The computer's previous aggressive personality was no longer present for some reason.